Synthesis of Cyclopentenes, Pyrroles, and Thiophenes via a

Nov 4, 2014 - Since the use of phosphorus ylides in 1953, reported by George Wittig, Wittig olefination has been employed extensively in organic synth...
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Synthesis of Cyclopentenes, Pyrroles, and Thiophenes via a Sequence of Propargyl−Allenyl Isomerizations, Michael Additions, and Intramolecular Wittig Reactions Guoqing Zhao, Qianyun Zhang, and Hongwei Zhou* Department of Chemistry, Zhejiang University (Campus Xixi), Hangzhou 310028, People’s Republic of China S Supporting Information *

ABSTRACT: Allenylphosphonium species, reactive Michael acceptors, could be generated from simple, readily available propargylphosphonium salts through propargyl−allenyl isomerization. We developed an efficient synthesis for cyclopentenes, pyrroles, and thiophenes via a sequence of propargyl−allenyl isomerizations, Michael additions, and intramolecular Wittig reactions.

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INTRODUCTION Five-membered cycles are common structural motifs found in a broad range of natural products, drug molecules, and useful materials.1−3 Among this group of compounds, cyclopentenes, pyrroles, and thiophenes are considered to be important targets because of intrinsic properties such as drug activity, photobiological activity, redox activity, and electron transport.4−6 Since the use of phosphorus ylides in 1953, reported by George Wittig, Wittig olefination has been employed extensively in organic syntheses.7 Correspondingly, the intramolecular Wittig reactions were applied in the formation of cyclic compounds.8 The classic strategy for the construction of the suitable ylide is the introduction of a carbonyl group and halide moiety into the same substrate and treatment of the substrate with triphenylphosphine/trialkylphosphine (Scheme 1a).9 In the 1960s, Schweizer continued to develop a general approach to cyclic compounds using vinylphosphonium salts as the Michael acceptors followed by an intramolecular Wittig reaction.10a−j McEwen applied Schweizer’s procedure to the further development of the synthesis of 1,2,5-trisubstituted pyrroles,10k and Murphy developed a domino reaction utilizing an in situ vinylphosphonium intermediate generated from the treatment of acetylene dicarboxylate with Ph3P10l (Scheme 1b). Recently, Lin’s group developed a series of intramolecular Wittig reactions via in situ-generated phosphorus ylides, using designed acceptors and suitable trapping reagents (Scheme 1c). During our study of propargyl−allenyl isomerization,11 we found that the allenylphosphonium species, which was generated in situ from base-promoted propargyl−allenyl isomerization of propargylphosphonium salts,11g could be attacked by mild nucleophiles. We anticipate that the allenylphosphonium species might react easily with a © XXXX American Chemical Society

nucleophile containing a carbonyl group to produce in situ an allyl ylide, which could undergo an intramolecular Wittig reaction to complete the cylization (Scheme 1d).

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RESULTS AND DISCUSSION

Stimulated by this proposal, we prepared propargylphosphonium bromide (1a) as the starting material, which could be synthesized readily via the treatment of (3-bromoprop-1ynyl)benzene with triphenylphosphine in toluene. We initiated our study by testing the reaction of 1a with 1,1-diethyl 2methylethane-1,1,2-tricarboxylate (2a) and examining the effects of base, solvent, and temperature. TEA (triethylamine), DIEA (N,N-diisopropylethylamine), and K2CO3 could initiate the propargyl−allenyl isomerization; NaH and KH could trigger the following Michael addition and Wittig reaction, affording cyclopentene (3a) as the sole product. At the same time, a significant solvent effect was observed (Table 1). The stereochemistry of 3a was established by a NOESY experiment, which clearly showed an NOE effect between the aromatic protons and the proton on the cyclic double bond. Under the optimized conditions, the scope of this reaction was further examined. The reacion was successful for various triphenyl(3-substituted prop-2-ynyl)phosphonium bromides. The R1 group can be an alkyl group or optionally substituted aryl group, and the carbonyl moiety, reacting intramolecularly with the ylide, can be an aliphatic ketone (Table 2, entries 3− 8), an aromatic ketone (Table 2, entries 9−11), or an ester (Table 2, entries 1 and 2). Received: August 13, 2014

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dx.doi.org/10.1021/jo501867h | J. Org. Chem. XXXX, XXX, XXX−XXX

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Table 2. Synthesis of Cyclopentenesa

Scheme 1. Proposed Cyclization via Propargyl−Allenyl Isomerization, Michael Addition, and Intramolecular Wittig Reaction

entry

R1

R2/R3

R4

3/yield (%)

1 2 3 4 5 6 7 8 9 10 11

p-Tol p-CI-C6H4 p-Tol Ph n-C4H9 n-C4H9 p-Tol Ph p-Tol p-Cl-C6H4 n-C4H9

CO2Et/CO2Et CO2Et/CO2Et CO2Et/CO2Et CO2Et/CO2Et CO2Et/CO2Et CO2Et/MeCO CO2Et/MeCO CO2Et/MeCO CN/CN CN/CN CN/CN

OMe OMe Me Me Me Me Me Me Ph Ph Ph

3a/78 3b/55 3c/61 3d/57 3e/67 3f/73 3g/83 3h/81 3i/62 3j/54 3k/59

a

The reaction was conducted using 1 (0.5 mmol), Et3N (1.5 mmol), 2 (0.4 mmol), and NaH (0.48 mmol) in toluene (3 mL) under N2.

might undergo an aromatization to give highly substituted pyrroles. The expected reactions proceeded smoothly under similar conditions and afforded pyrroles in moderate to good yields (Table 3). Generally, a sulfur anion is a better nucleophile for the Michael addition than a nitrogen anion; it is a reasonable inference that the thiophenes could be synthesized via this strategy. The experimental results proved this out, and the thiophenes were obtained in moderate yields (Table 4). Notably, these reactions could not offer any isolatable products when only the second base NaH was charged, indicating that the presence of triethylamine is essential for triggering the propargyl−allenyl isomerization. We therefore propose the plausible mechanism shown in Scheme 2. First, the base-assisted propargyl−allenyl isomerization of triphenyl(prop-2-ynyl)phosphonium salt gives triphenyl allenylphosphonium salt,11g,12 which is attacked by the nucleophile to afford in situ an allyl ylide. A subsequent Wittig process yields the cyclopentenes, 2-methylene-2,5-dihydropyrroles, or 2-methylene-2,5-dihydrothiophenes; the latter two species may undergo an aromatization to afford pyrroles or thiophenes. As a simple route to five-membered cycles, the usefulness of this protocol should be demonstrated on a gram-scale preparation. We thus conducted the reaction of 1a with 2a on a 2 g scale and obtained 3a in 72% yield (Scheme 3).

Table 1. Base, Solvent, and Temperature Effectsa

entry

base 1

base 2

solvent

T (°C)

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

t-BuOK DBU DBN DIEA DIEA K2CO3 TEA TEA TEA DIEA TEA TEA TEA TEA TEA TEA

t-BuOK NaH NaH NaH LiH NaH NaH NaH NaH NaH NaH KH LiH NaH NaH NaH

toluene toluene toluene toluene toluene toluene toluene MeCN 1,4-dioxane THF THF toluene toluene toluene toluene toluene

rt rt rt rt rt rt rt rt rt rt rt rt rt 40 60 80

0 7 6 45 20 48 62 0 trace 8 12 60 32 78 72 74

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CONCLUSION In summary, we have reported an efficient synthesis for cyclopentenes, pyrroles, and thiophenes via a sequence of propargyl−allenyl isomerizations, Michael additions, and intramolecular Wittig reactions. Further studies of the applications and expansions of this reaction are ongoing in our laboratory.

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a

The reaction was conducted using 1a (0.5 mmol), base 1 (1.5 mmol), 2a (0.4 mmol), and base 2 (0.48 mmol) in solvent (3 mL) under N2.

EXPERIMENTAL SECTION

Typical Procedure for the Synthesis of 3a (0.5 mmol scale). To a solution of 1a (236 mg, 0.5 mmol) and Et3N (152 mg, 1.5 mmol) in dry toluene (2.0 mL) at room temperature was added a mixture of 2a (93 mg, 0.4 mmol) and NaH (16 mg, 0.48 mmol, w = 70%) in dry toluene (1.0 mL). The reaction mixture was stirred at 40 °C for 12 h. Upon completion, the reaction was quenched with a saturated aqueous

Then our attention was diverted to the synthesis of pyrroles. The reactive allenylphosphonium intermediate might be attacked by a nitrogen anion, and the desired cyclic products B

dx.doi.org/10.1021/jo501867h | J. Org. Chem. XXXX, XXX, XXX−XXX

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Table 3. Synthesis of Pyrrolesa

Scheme 2. Plausible Mechanism

Scheme 3. Gram-Scale Preparation

silica gel (50:1 petroleum ether/ethyl acetate) of the reaction mixture afforded 3a as a yellow amorphous solid (107 mg, 78%). Typical Procedure for the Synthesis of 3a (2 g scale). To a solution of 1a (5.6 g, 12 mmol) and Et3N (3.6 g, 36 mmol) in dry toluene (30 mL) at room temperature was added a mixture of 2a (2.3 g, 10 mmol) and NaH (0.4 g, 12 mmol, w = 70%) in dry toluene (30 mL). The reaction mixture was stirred at 40 °C for 18 h. Upon completion, the reaction was quenched with a saturated aqueous NH4Cl solution, and the mixture was extracted with ethyl acetate and dried over anhydrous Na2SO4. After evaporation, chromatography on silica gel (50:1 petroleum ether/ethyl acetate) of the reaction mixture afforded 3a as a yellow amorphous solid (2.5 g, 72%). (E)-Diethyl 4-Methoxy-2-(4-methylbenzylidene)cyclopent-3-ene1,1-dicarboxylate (3a). Yellow amorphous solid (107 mg, 78% yield): 1H NMR (400 MHz, CDCl3) δ 7.23 (d, J = 8.0 Hz, 2 H), 7.13 (d, J = 8.0 Hz, 2 H), 6.34 (s, 1 H), 5.75 (s, 1 H), 4.23−4.28 (m, 4 H), 3.73 (s, 3 H), 3.21 (s, 2 H), 2.33 (s, 3 H), 1.29 (t, J = 7.6 Hz, 6 H); 13 C NMR (100 MHz, CDCl3) δ 170.1, 166.7, 139.7, 135.7, 135.6, 128.9, 127.9, 119.6, 97.5, 61.8, 61.3, 57.6, 39.6, 21.1, 14.0; IR (neat) 1727, 1237, 1176, 1093, 1042 cm−1; HRMS (EI-TOF) calcd for C20H24O5 (M+) 344.1624, found 344.1616. (E)-Diethyl 2-(4-Chlorobenzylidene)-4-methoxycyclopent-3-ene1,1-dicarboxylate (3b). Yellow oil (80 mg, 55% yield): 1H NMR (400 MHz, CDCl3) δ 7.25−7.29 (m, 4 H), 6.32 (s, 1 H), 5.71 (s, 1 H), 4.26−4.31 (q, J = 6.7 Hz, 4 H), 3.77 (s, 3 H), 3.23 (s, 2 H), 1.32 (t, J = 6.0 Hz, 6 H); 13C NMR (100 MHz, CDCl3) δ 169.9, 167.8, 141.2, 137.1, 131.4, 129.3, 128.4, 118.3, 97.2, 61.9, 61.4, 57.7, 39.7, 14.0; IR (neat) 1730, 1616, 1361, 1240, 1094 cm−1; HRMS (EI-TOF) calcd for C19H21ClO5 (M+) 364.1078, found 364.1071. (E)-Diethyl 4-Methyl-2-(4-methylbenzylidene)cyclopent-3-ene1,1-dicarboxylate (3c). Amorphous solid (80 mg, 61% yield): 1H NMR (400 MHz, CDCl3) δ 7.17 (t, J = 7.0 Hz, 2 H), 7.05 (d, J = 8.0 Hz, 2 H), 6.40 (s, 1 H), 6.31 (s, 1 H), 4.14−4.20 (m, 4 H), 3.03 (s, 2 H), 2.26 (s, 3 H), 1.84 (s, 3 H), 1.20 (t, J = 7.2 Hz, 6 H); 13C NMR (100 MHz, CDCl3) δ 170.6, 147.7, 142.7, 136.1, 135.3, 128.9, 128.3, 124.8, 122.3, 63.7, 61.6, 45.0, 21.1, 17.1, 14.0; IR (neat) 2911, 1729, 1443, 1240, 1177, 1067 cm−1; HRMS (EI-TOF) calcd for C20H24O4 (M+) 328.1675, found 328.1680.

a

The reaction was conducted using 1 (0.5 mmol), Et3N (1.5 mmol), 4 (0.4 mmol), and NaH (0.48 mmol) in toluene (3 mL) under N2.

Table 4. Synthesis of Thiophenesa

entry

R1

R2/R3

7/yield (%)

1 2 3 4 5 6 7 8

Ph Ph p-Tol p-Tol p-Cl-C6H4 p-CI-C6H4 n-C4Hg n-C4H9

H/OMe Me/OEt H/OMe Me/OEt H/OMe Me/OEt H/OMe Me/OEt

7a/65 7b/54 7c/57 7d/54 7e/63 7f/55 7g/63 7h/55

a

The reaction was conducted using 1 (0.5 mmol), Et3N (1.5 mmol), 6 (0.4 mmol), and NaH (0.48 mmol) in toluene (3 mL) under N2.

NH4Cl solution, and the mixture was extracted with ethyl acetate and dried over anhydrous Na2SO4. After evaporation, chromatography on C

dx.doi.org/10.1021/jo501867h | J. Org. Chem. XXXX, XXX, XXX−XXX

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Article

(E)-Diethyl 2-Benzylidene-4-methylcyclopent-3-ene-1,1-dicarboxylate (3d). Yellow oil (72 mg, 57% yield): 1H NMR (400 MHz, CDCl3) δ 7.29−7.35 (m, 4 H), 7.27−7.31 (m, 1 H), 6.51 (s, 1 H), 6.40 (s, 1 H), 4.22−4.28 (m, 4 H), 3.12 (s, 2 H), 1.92 (s, 3 H), 1.29 (t, J = 7.2 Hz, 6 H); 13C NMR (100 MHz, CDCl3) δ 170.5, 148.3, 143.4, 138.2, 128.3, 128.2, 126.4, 124.7, 122.3, 63.7, 61.7, 45.0, 17.1, 14.0; IR (neat) 2938, 1726, 1242, 1129 cm−1; HRMS (EI-TOF) calcd for C19H22O4 (M+) 314.1518, found 314.1522. (E)-Diethyl 4-Methyl-2-pentylidenecyclopent-3-ene-1,1-dicarboxylate (3e). Yellow oil (79 mg, 67% yield): 1H NMR (400 MHz, CDCl3) δ 6.00 (s, 1 H), 5.45 (t, J = 7.6 Hz, 1 H), 4.17−4.22 (m, 4 H), 3.03 (s, 2 H), 2.17 (q, J = 7.1 Hz, 2 H), 1.86 (s, 3 H), 1.31−1.4 (m, 2 H), 1.23−1.27 (m, 8 H), 0.88 (t, J = 7.2 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 170.9, 143.8, 141.7, 123.9, 123.6, 62.5, 61.5, 45.2, 31.9, 29.2, 22.2, 16.8, 14.0, 13.9; IR (neat) 2960, 2931, 1731, 1460, 1369, 1260 cm−1; HRMS (EI-TOF) calcd for C17H26O4 (M+) 294.1831, found 294.1827. (E)-Ethyl 1-Acetyl-4-methyl-2-pentylidenecyclopent-3-enecarboxylate (3f). Yellow oil (77 mg, 73% yield): 1H NMR (400 MHz, CDCl3) δ 6.00 (s, 1 H), 5.38 (t, J = 7.6 Hz, 1 H), 4.17−4.23 (m, 2 H), 3.12 (d, J = 17.6 Hz, 1 H), 2.79 (d, J = 17.6 Hz, 1 H), 2.13−2.19 (m, 5 H), 1.86 (s, 3 H), 1.29−1.39 (m, 4 H), 1.25 (t, J = 7.2 Hz, 3 H), 0.88 (t, J = 7.0 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 203.3, 171.2, 144.5, 142.1, 124.2, 123.8, 69.4, 61.4, 43.9, 31.8, 29.2, 25.6, 22.2, 16.8, 13.98, 13.89; IR (neat) 2959, 2932, 1710, 1444, 1359, 1245 cm−1; HRMS (EI-TOF) calcd for C16H24O3 (M+) 264.1725, found 264.1734. (E)-Ethyl 1-Acetyl-4-methyl-2-(4-methylbenzylidene)cyclopent-3enecarboxylate (3g). Yellow oil (99 mg, 83% yield): 1H NMR (400 MHz, CDCl3) δ 7.24 (d, J = 8.0 Hz, 2 H), 7.14 (d, J = 8.0 Hz, 2 H), 6.44 (s, 1 H), 6.41 (s, 1 H), 4.24−4.30 (m, 2 H), 3.25 (d, J = 18.0 Hz, 1 H), 2.85 (d, J = 18.4 Hz, 1 H), 2.34 (s, 3 H), 2.25 (s, 3 H), 1.94 (s, 3 H), 1.31 (t, J = 7.2 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 203.0, 170.7, 148.5, 142.9, 136.3, 135.0, 129.0, 128.1, 125.2, 122.3, 70.5, 61.6, 43.6, 25.7, 21.1, 17.1, 14.0; IR (neat) 2979, 1711, 1442, 1356, 1243, 1097 cm−1; HRMS (EI-TOF) calcd for C19H22O3 (M+) 298.1569, found 298.1566. (E)-Ethyl 1-Acetyl-2-benzylidene-4-methylcyclopent-3-enecarboxylate (3h). Yellow oil (92 mg, 81% yield): 1H NMR (400 MHz, CDCl3) δ 7.33 (d, J = 4.8 Hz, 4 H), 7.20−7.23 (m, 1 H), 6.44 (s, 2 H), 4.24−4.30 (m, 2 H), 3.25 (d, J = 18.4 Hz, 1 H), 2.85 (d, J = 18.4 Hz, 1 H), 2.25 (s, 3 H), 1.94 (s, 3 H), 1.31 (t, J = 7.2 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 202.8, 170.7, 149.0, 143.7, 137.9, 128.3, 128.2, 126.6, 125.1, 122.4, 70.5, 61.6, 43.6, 25.7, 17.1, 14.0; IR (neat) 2979, 1710, 1444, 1356, 1243, 1217 cm−1; HRMS (EI-TOF) calcd for C18H20O3 (M+) 284.1412, found 284.1409. (E)-2-(4-Methylbenzylidene)-4-phenylcyclopent-3-ene-1,1-dicarbonitrile (3i). Yellow oil (73 mg, 62% yield): 1H NMR (400 MHz, CDCl3) δ 7.37−7.46 (m, 5 H), 7.33 (d, J = 8.4 Hz, 2 H), 7.23 (d, J = 8.4 Hz, 2 H), 7.17 (s, 1 H), 6.83 (s, 1 H), 3.71 (s, 2 H), 2.39 (s, 3 H); 13 C NMR (100 MHz, CDCl3) δ 145.4, 138.6, 138.4, 132.9, 132.7, 129.7, 129.5, 128.9, 128.5, 126.0, 125.9, 122.3, 115.9, 44.2, 37.1, 21.3; IR (neat) 1496, 1445, 1264, 1080, 1031 cm−1; HRMS (EI-TOF) calcd for C21H16N2 (M+) 296.1313, found 296.1310. (E)-2-(4-Chlorobenzylidene)-4-phenylcyclopent-3-ene-1,1-dicarbonitrile (3j). Amorphous solid (68 mg, 54% yield): 1H NMR (400 MHz, CDCl3) δ 7.46−7.49 (m, 2 H), 7.35−7.45 (m, 7 H), 7.12 (s, 1 H), 6.81 (s, 1 H), 3.74 (s, 2 H); 13C NMR (100 MHz, CDCl3) δ 146.8, 139.9, 134.3, 134.0, 132.7, 130.1, 129.7, 129.0, 126.1, 124.5, 121.7, 115.6, 44.3, 37.1; IR (neat) 1737, 1488, 1442, 1265, 1086 cm−1; HRMS (EI-TOF) calcd for C20H13ClN2 (M+) 316.0767, found 316.0767. (E)-2-Pentylidene-4-phenylcyclopent-3-ene-1,1-dicarbonitrile (3k). Yellow oil (62 mg, 59% yield): 1H NMR (400 MHz, CDCl3) δ 7.37−7.46 (m, 5 H), 6.82 (d, J = 1.6 Hz, 1 H), 5.96 (t, J = 7.8 Hz, 1 H), 3.66 (s, 2 H), 2.30−2.36 (q, J = 7.6 Hz, 2 H), 1.47−1.52 (m, 2 H), 1.37−1.42 (m, 2 H), 0.95 (t, J = 7.4 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 142.7, 139.1, 133.1, 129.4, 128.9, 128.5, 125.8, 121.5, 116.1, 44.7, 35.9, 31.2, 29.3, 22.2, 13.8; IR (neat) 2958, 2928, 1495, 1344, 1261 cm−1; HRMS (EI-TOF) calcd for C18H18N2 (M+) 262.1470, found 262.1471.

2-Benzyl-4-ethoxy-1-tosyl-1H-pyrrole (5a). Yellow oil (85 mg, 60% yield): 1H NMR (400 MHz, CDCl3) δ 7.52−7.54 (m, 2 H), 7.19−7.21 (m, 5 H), 7.06 (dd, J1 = 8.0 Hz, J2 = 1.8 Hz, 2 H), 6.73 (d, J = 2.0 Hz, 1 H), 5.54−5.55 (m, 1 H), 4.03 (s, 2 H), 3.85 (q, J = 6.9 Hz, 2 H), 2.39 (s, 3 H), 1.33 (t, J = 7.0 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 149.1, 144.4, 138.0, 136.2, 134.0, 129.7, 129.0, 128.3, 126.6, 126.3, 107.9, 102.2, 65.8, 33.7, 21.5, 14.7; IR (neat) 2928, 1743, 1614, 1166, 1086 cm−1; HRMS (EI-TOF) calcd for C20H21NO3S (M+) 355.1242, found 355.1239. 4-Ethoxy-2-(4-methylbenzyl)-1-tosyl-1H-pyrrole (5b). Yellow oil (81 mg, 55% yield): 1H NMR (400 MHz, CDCl3) δ 7.54 (d, J = 8.4 Hz, 2 H), 7.20 (d, J = 8.0 Hz, 2 H), 7.01 (d, J = 8.0 Hz, 2 H), 6.94 (d, J = 8.0 Hz, 2 H), 6.73 (d, J = 2.0 Hz, 1 H), 5.54−5.55 (m, 1 H), 3.99 (s, 2 H), 3.85 (q, J = 7.0 Hz, 2 H), 2.40 (s, 3 H), 2.31 (s, 3 H), 1.33 (t, J = 7.0 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 149.0, 144.3, 136.3, 135.9, 134.9, 134.4, 129.7, 128.94, 128.86, 126.5, 107.8, 102.2, 65.7, 33.3, 21.5, 21.0, 14.7; IR (neat) 2921, 1743, 1701, 1364, 1166, 1087 cm−1; HRMS (EI-TOF) calcd for C21H23NO3S (M+) 369.1399, found 369.1402. 2-(4-Chlorobenzyl)-4-ethoxy-1-tosyl-1H-pyrrole (5c). Yellow oil (82 mg, 53% yield): 1H NMR (400 MHz, CDCl3) δ 7.45−7.47 (m, 2 H), 7.17 (d, J = 8.4 Hz, 2 H), 7.12−7.14 (m, 2 H), 6.94−6.96 (m, 2 H), 6.75 (d, J = 2.0 Hz, 1 H), 5.61−5.62 (m, 1 H), 4.00 (s, 2 H), 3.87 (q, J = 7.1 Hz, 2 H), 2.39 (s, 3 H), 1.34 (t, J = 7.0 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 148.9, 144.5, 136.5, 136.1, 133.0, 132.2, 130.2, 129.7, 128.3, 126.4, 108.1, 102.5, 65.8, 32.9, 21.5, 14.6; IR (neat) 1742, 1703, 1364, 1166, 1067 cm −1 ; HRMS (EI-TOF) calcd for C20H20ClNO3S (M+) 389.0852, found 389.0847. 3-Ethoxy-2-methyl-5-pentyl-1-tosyl-1H-pyrrole (5d). Yellow oil (81 mg, 58% yield): 1H NMR (400 MHz, CDCl3) δ 7.49 (d, J = 8.4 Hz, 2 H), 7.24 (d, J = 8.4 Hz, 2 H), 5.86 (s, 1 H), 3.88 (q, J = 6.9 Hz, 2 H), 2.76 (t, J = 7.6 Hz, 2 H), 2.38 (s, 3 H), 2.25 (s, 3 H), 1.57−1.61 (m, 2 H), 1.25−1.33 (m, 7 H), 0.88 (t, J = 7.2 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 145.0, 144.1, 137.4, 135.3, 129.8, 125.9, 116.0, 104.1, 67.1, 31.5, 29.0, 28.9, 22.5, 21.5, 15.1, 14.0, 10.8; IR (neat) 2958, 2923, 1701, 1357, 1162, 1090 cm−1; HRMS (EI-TOF) calcd for C19H27NO3S (M+) 349.1712, found 349.1716. 4-Ethoxy-2-pentyl-1-tosyl-1H-pyrrole (5e). Yellow oil (83 mg, 62% yield): 1H NMR (400 MHz, CDCl3) δ 7.62 (d, J = 8.4 Hz, 2 H), 7.26 (d, J = 8.0 Hz, 2 H), 6.68 (d, J = 2.0 Hz, 1 H), 5.76 (m, 1 H), 3.87 (q, J = 6.9 Hz, 2 H), 2.62 (t, J = 7.8 Hz, 2 H), 2.39 (s, 3 H), 1.49−1.54 (m, 2 H), 1.35 (t, J = 7.2 Hz, 3 H), 1.25−1.28 (m, 4 H), 0.84−0.87 (m, 3 H); 13C NMR (100 MHz, CDCl3) δ 149.2, 144.4, 136.5, 135.8, 129.8, 126.4, 105.7, 101.7, 65.7, 31.3, 28.3, 27.4, 22.4, 21.5, 14.7, 13.9; IR (neat) 2929, 2870, 1705, 1597, 1347 cm−1; HRMS (EI-TOF) calcd for C18H25NO3S (M+) 335.1555, found 335.1558. 4-Ethoxy-2-neopentyl-1-tosyl-1H-pyrrole (5f). Yellow oil (74 mg, 55% yield): 1H NMR (400 MHz, CDCl3) δ 7.59 (d, J = 7.6 Hz, 2 H), 7.23 (d, J = 8.0 Hz, 2 H), 6.64 (d, J = 2.0 Hz, 1 H), 5.78 (d, J = 2.0 Hz, 1 H), 3.85 (q, J = 6.9 Hz, 2 H), 2.67 (s, 2 H), 2.38 (s, 3 H), 1.34 (t, J = 7.0 Hz, 3 H), 0.93 (s, 9 H); 13C NMR (100 MHz, CDCl3) δ 149.6, 144.3, 136.4, 134.0, 129.6, 126.3, 109.0, 102.3, 65.7, 39.7, 32.2, 29.3, 21.5, 14.6; IR (neat) 2958, 1705, 1596, 1363, 1164, 1085 cm−1; HRMS (EI-TOF) calcd for C18H25NO3S (M+) 335.1555, found 335.1551. 2-Benzyl-3-ethoxy-1-(methylsulfonyl)-5-pentyl-1H-pyrrole (5g). Yellow oil (74 mg, 53% yield): 1H NMR (400 MHz, CDCl3): δ (ppm) 7.22−7.26 (m, 4 H), 7.14−7.17 (m, 1 H), 5.99 (s, 1 H), 4.15 (s, 2 H), 4.01 (q, J = 7.1 Hz, 2 H), 2.71 (t, J = 7.8 Hz, 2 H), 2.43 (s, 3 H), 1.59−1.64 (m, 2 H), 1.33−1.38 (m, 7 H), 0.88−0.92 (m, 3 H); 13 C NMR (100 MHz, CDCl3): δ (ppm) 145.9, 140.4, 135.1, 128.7, 128.2, 125.9, 118.2, 103.1, 66.9, 41.4, 31.5, 29.6, 28.9, 28.8, 22.5, 15.3, 14.0; IR (neat): 2930, 2969, 1728, 1636, 1356, 1155 cm−1; HRMS (EITOF) calcd for C19H27NO3S (M+): 349.1712, found: 349.1702. 5-Benzylidene-2,2-dimethyl-1-tosyl-2,5-dihydro-1H-pyrrole (1.2:1 E:Z) (5h). Gum (106 mg, 72% yield): 1H NMR (400 MHz, CDCl3) δ 7.77−7.79 (m, 4 H), 7.30−7.38 (m, 8 H), 7.20−7.25 (m, 4 H), 6.50 (d, J = 7.6 Hz, 0.8 H), 6.45 (d, J = 7.6 Hz, 1 H), 5.72 (d, J = 7.2 Hz, 0.8 H), 5.63 (d, J = 7.6 Hz, 1 H), 5.23 (d, J = 8.4 Hz, 1.8 H), 2.35 (s, 5.5 H), 1.48 (s, 6 H), 1.42 (s, 5 H); 13C NMR (100 MHz, CDCl3) δ 160.63, 160.60, 157.8, 157.6, 143.03, 143.00, 139.8, 139.7, 136.3, D

dx.doi.org/10.1021/jo501867h | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Article

HRMS (EI-TOF) calcd for C12H20OS (M+) 212.1235, found 212.1232.

136.1, 129.5, 129.4, 128.7, 128.6, 128.20, 128.16, 127.8, 127.7, 127.3, 127.2, 115.5, 115.2, 109.9, 109.8, 57.6, 57.5, 28.7, 28.4, 21.44, 21.40; IR (neat) 1569, 1323, 1147, 1093 cm−1; HRMS (EI-TOF) calcd for C20H21NO2S (M+) 339.1293, found 339.1302. 5-(4-Chlorobenzylidene)-2,2-dimethyl-1-tosyl-2,5-dihydro-1Hpyrrole (2:1 E:Z) (5i). Yellow oil (116 mg, 78% yield): 1H NMR (400 MHz, CDCl3) δ 7.76−7.79 (m, 3 H), 7.22−7.30 (m, 9 H), 6.45 (d, J = 7.6 Hz, 0.5 H), 6.41 (d, J = 7.6 Hz, 1 H), 5.77 (d, J = 7.2 Hz, 0.5 H), 5.67 (d, J = 7.6 Hz, 1 H), 5.08 (br, 1.5 H), 2.37 (s, 4.5 H), 1.48 (s, 6 H), 1.42 (s, 3 H); 13C NMR (100 MHz, CDCl3) δ 161.0, 157.6, 143.2, 143.1, 139.8, 139.7, 134.8, 134.0, 132.6, 129.51, 129.48, 129.0, 128.9, 128.86, 128.81, 128.7, 127.3, 127.2, 116.5, 116.1, 108.6, 108.5, 57.7, 57.5, 28.6, 28.4, 21.5; IR (neat) 1595, 1491, 1324, 1152, 1092 cm−1; HRMS (EI-TOF) calcd for C20H20ClNO2S (M+) 373.0903, found 373.0910. 2-Benzyl-4-methoxythiophene (7a). Yellow oil (53 mg, 65% yield): 1H NMR (400 MHz, CDCl3) δ 7.27−7.31 (m, 2 H), 7.21− 7.23 (m, 3 H), 6.47−6.48 (m, 1 H), 6.02 (d, J = 1.6 Hz, 1 H), 4.02 (s, 2 H), 3.73 (s, 3 H); 13C NMR (100 MHz, CDCl3) δ 157.7, 143.2, 139.8, 128.6, 128.5, 126.5, 117.8, 94.8, 56.9, 36.6; IR (neat) 1562, 1480, 1390, 1203, 1149 cm−1; HRMS (EI-TOF) calcd for C12H12OS (M+) 204.0609, found 204.0613. 5-Benzyl-3-ethoxy-2-methylthiophene (7b). Yellow oil (50 mg, 54% yield): 1H NMR (400 MHz, CDCl3) δ 7.21−7.24 (m, 2 H), 7.15−7.17 (m, 3 H), 6.42 (s, 1 H), 3.93 (s, 2 H), 3.88 (q, J = 6.9 Hz, 2 H), 2.13 (s, 3 H), 1.24 (t, J = 7.0 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 151.6, 140.2, 137.9, 128.6, 128.5, 126.5, 116.5, 115.6, 67.4, 36.7, 15.3, 10.6; IR (neat) 2977, 2919, 1577, 1493, 1449 cm−1; HRMS (EI-TOF) calcd for C14H16OS (M+) 232.0922, found 232.0924. 4-Methoxy-2-(4-methylbenzyl)thiophene (7c). Yellow oil (50 mg, 57% yield): 1H NMR (400 MHz, CDCl3) δ 7.13−7.17 (m, 4 H), 6.51 (s, 1 H), 6.06 (s, 1 H), 4.03 (s, 2 H), 3.77 (s, 3 H), 2.36 (s, 3 H); 13C NMR (100 MHz, CDCl3) δ 157.7, 143.6, 136.7, 136.0, 129.2, 128.5, 117.6, 94.7, 56.9, 36.2, 21.0; IR (neat) 1562, 1513, 1480, 1369, 1203 cm−1; HRMS (EI-TOF) calcd for C13H14OS (M+) 218.0765, found 218.0768. 3-Ethoxy-2-methyl-5-(4-methylbenzyl)thiophene (7d). Yellow oil (53 mg, 54% yield): 1H NMR (400 MHz, CDCl3) δ 7.12−7.16 (m, 4 H), 6.51 (s, 1 H), 3.95−4.01 (m, 4 H), 2.35 (s, 3 H), 2.23 (s, 3 H), 1.34 (t, J = 7.0 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 151.6, 138.3, 137.1, 136.0, 129.2, 128.4, 116.4, 115.5, 67.3, 36.3, 21.0, 15.3, 10.6; IR (neat) 2977, 2917, 1577, 1513, 1362, 1399 cm−1; HRMS (EI-TOF) calcd for C15H18OS (M+) 246.1078, found 246.1077. 2-(4-Chlorobenzyl)-4-methoxythiophene (7e). Yellow oil (60 mg, 63% yield): 1H NMR (400 MHz, CDCl3) δ 7.17−7.20 (m, 2 H), 7.07−7.09 (m, 2 H), 6.38−6.39 (m, 1 H), 5.96−5.97 (m, 1 H), 3.92 (s, 2 H), 3.67 (s, 3 H); 13C NMR (100 MHz, CDCl3) δ 157.8, 142.5, 138.3, 132.4, 129.9, 128.6, 118.0, 95.1, 57.0, 36.0; IR (neat) 1635, 1481, 1440, 1202 cm−1; HRMS (EI-TOF) calcd for C12H11ClOS (M+) 238.0219, found 238.0216. 5-(4-Chlorobenzyl)-3-ethoxy-2-methylthiophene (7f). Yellow oil (58 mg, 55% yield): 1H NMR (400 MHz, CDCl3) δ 7.18−7.20 (m, 2 H), 7.07−7.09 (m, 2 H), 6.40 (s, 1 H), 3.86−3.91 (m, 4 H), 2.14 (s, 3 H), 1.24 (t, J = 7.0 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 151.6, 138.6, 137.2, 132.3, 129.9, 128.6, 116.7, 115.9, 67.4, 36.1, 15.3, 10.6; IR (neat) 2975, 2919, 1577, 1489, 1384 cm−1; HRMS (EI-TOF) calcd for C14H15ClOS (M+) 266.0532, found 266.0536. 4-Methoxy-2-pentylthiophene (7g).13 Yellow oil (46 mg, 63% yield): 1H NMR (400 MHz, CDCl3) δ 6.47 (m, 1 H), 6.01 (d, J = 1.6 Hz, 1 H), 3.78 (s, 3 H), 2.69−2.73 (m, 2 H), 1.64−1.67 (m, 2 H), 1.33−1.37 (m, 4 H), 0.89−0.93 (m, 3 H); 13C NMR (100 MHz, CDCl3) δ 157.7, 145.0, 116.6, 93.6, 56.9, 31.2, 30.9, 30.5, 22.4, 14.0; IR (neat) 2929, 2856, 1564, 1459, 1391, 1204 cm−1; HRMS (EI-TOF) calcd for C10H16OS (M+) 184.0922, found 184.0927. 3-Ethoxy-2-methyl-5-pentylthiophene (7h). Yellow oil (47 mg, 55% yield): 1H NMR (400 MHz, CDCl3) δ 6.47 (s, 1 H), 3.98 (q, J = 7.1 Hz, 2 H), 2.66 (t, J = 7.6 Hz, 2 H), 2.23 (s, 3 H), 1.58−1.64 (m, 2 H), 1.31−1.36 (m, 7 H), 0.90 (t, J = 7.0 Hz, 3 H); 13C NMR (100 MHz, CDCl3) δ 151.4, 139.7, 115.4, 114.3, 67.3, 31.2, 31.1, 30.6, 22.4, 15.4, 14.0, 10.6; IR (neat) 2926, 2858, 1579, 1458, 1383, 1150 cm−1;

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ASSOCIATED CONTENT

S Supporting Information *

Proton and carbon NMR spectra of products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+ 86)-571-8892-0271. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (20972134) and the Natural Science Foundation of Zhejiang Province (LY14B020008) is greatly appreciated.

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REFERENCES

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